Fierce artillery, tank and rocket battles raged Wednesday around a key military base here that is the last major obstacle in the north's advance toward the southern stronghold of Aden.
Fighting around the Al Anad base, 35 miles northwest of Aden, has gone on for days in what could be the decisive battle of Yemen's two-week civil war.
The prize in the seesaw battle is control of the country's main north-south road and the gateway to Aden, on the tip of the Arabian Peninsula.
The fighting broke out May 5 after months of small-scale skirmishes between the rival leaderships of the former North and South Yemen, which merged in 1990.
Northern-controlled Sana radio appealed Wednesday to the estimated 250,000 people living in Aden to declare their loyalty to the north under President Ali Abdullah Saleh.
"This is the last chance for those who are misled by the forces of apostasy and secession to join the ranks of their brothers," the broadcast said.
The southern command said its forces had crushed an attempt to capture Al Anad and routed a northern brigade 30 miles east of Aden. It said 400 northern soldiers were killed but did not mention its own casualties.
Northern forces said they had captured Al Anad on Monday, but their commanders told reporters Wednesday that a sprawling air base south of the main Al Anad complex was still in southern hands.